The present invention relates to a fixing apparatus which fixes the toner image on transfer medium by welding the toner image on recording medium to the recording medium.
Usually, an eloctrophotographic image forming apparatus is equipped with a fixing apparatus comprising a heating means (roller, endless belt, etc.) and a pressing means (roller, endless belt, etc.). The heating and pressing means of the fixing apparatus are rotationally driven while being kept pressed upon each other, and while transfer medium, to which toner composed of resinous substance, magnetic substance, coloring agent, etc is held, is conveyed through the contact area (nipping portion) between the heating and pressing means, remaining nipped by the heating and pressing means, heat and pressure are applied to the transfer medium from the heating and pressing means. As a result, the toner is welded (fixed) to the transfer medium
Regarding a fixing apparatus such as the one described above, Japanese Laid-Open U.M. Application 51-109736 disclose a heat generating method which uses the Joule effect. According to this method, the heating means comprises an excitation coil, and an electrically conductive layer (which hereinafter will be simply referred to an conductive layer) on the inward surface of the fixing roller head, and heat is generated as eddy current is induced in the conductive layer by the excitation coil. This method is characterized in that it makes it possible to place a heat generating source very close to toner, being therefore capable of substantially reducing the time it takes for the fixing roller surface temperature to reach the proper temperature for fixation when starting up the fixing apparatus, compared to a conventional heat roller type heating method which employs a halogen lamp. Further, it is also characterized in that it is shorter and simpler in the heat transmission path from the heat generating source to the toner, being therefor higher in thermal efficiency.
In a fixing apparatus employing an electromagnetic induction based heating method, a strong magnetic field cannot be obtained unless the distance between the excitation coil and the electrically conductive layer on the internal surface of the fixing roller is made as small as possible. Therefore, the excitation coil must be placed as close as possible to the fixing roller without allowing the excitation coil to come into contact with the fixing roller while the fixing roller is rotating. Also, the fixing roller of a fixing apparatus must be enabled to maintain its temperature at a level necessary to fix the toner on the recording medium. Thus, the excitation coil is positioned close to the fixing roller in a manner to oppose the fixing roller. Therefore, the temperature of the excitation coil increases as the temperature of the fixing roller increases. As the excitation coil increases in temperature, its electrical resistance increases, reducing the current which is allowed to flow through it, which in turn reduces the heat it generates. In other words, as the excitation coil is heated, it declines in its heat generation efficiency, which is a problem. Further, usually, the excitation coil is disposed within a coil unit which comprises the excitation coil and the member for supporting the excitation coil. Therefore, it is difficult for the heat from the excitation coil to directly dissipate into the ambience. Thus, in order to provide a reliable fixing apparatus, it is necessary to solve this problem, that is, the increase in the excitation coil temperature. As for the countermeasure therefor, it is possible to place a piece of non-magnetic substance, such as a resinous member, in contact with the excitation coil in order to transfer the heat of the excitation coil to the piece of non-magnetic substance. However, when the distance between the coil unit and fixing roller is small, the coil unit is affected by the heat from the fixing roller, preventing sometimes the heat from the excitation coil from being dissipated. Thus, an efficient method for dissipating heat from the coil unit has been desired.
As for the methods for preventing the increase in the excitation coil temperature, there has been devised a method which employs a heat transferring member formed of efficient heat conductor such as aluminum, copper, or the like, to outwardly transfer the heat from the fixing roller, a method which employs a cooling fan to air cool the excitation coil, and the like method. In the case of the method which employs the cooling fan, air passages are provided within the excitation coil unit as disclosed in Japanese Laid-Open Patent Application No. 54-39645.
The internal positioning of a heat transferring member formed of efficient heat conductor such as aluminum, copper, or the like, however, increases the overall thermal capacity of the internal members of the fixing roller. As a result, not only does the start-up time increases, but also the electrical power necessary for the satisfactory fixation. A method employing a cooling fan also suffers from these problems. Therefore, there is demand for a structural arrangement which prevents the increase in the excitation coil temperature without increasing the electrical power necessary for the satisfactory fixation.
The primary object of the present invention is to provide a fixing apparatus in which the temperature of the excitation coil does not increases and the electrical power necessary for fixation is minimized, and also to provide an image forming apparatus comprising such a fixing apparatus
According to one of the characteristic aspects of the present invention, a fixing apparatus, or an image forming apparatus, comprises: a coil for generating a magnetic field; a heating medium in which heat is generated as eddy current is induced therein by the magnetic field generated by the coil, and which thermally fixes the image on recording medium with the use of the generated heat; a non-magnetic member disposed between the heat generating portion of the heating medium and the coil, with the provision of a predetermined gap from the heating medium, the coil being placed in contact with the nonmagnetic member, and the portion of the non-magnetic member in contact with the coil extending outward of the heating medium.
According to another characteristic aspects of the present invention, a fixing apparatus or an image forming apparatus, comprises: a coil for generating a magnetic field; a fixing roller in which heat is generated as eddy current is induced therein by the magnetic field generated by the coil, and which fixes the unfixed toner image on recording medium with the use of the generated heat; a non-magnetic member disposed between the heat generating portion of the heating medium and the coil, with the provision of a predetermined gap from the heating medium, the coil being placed in contact with the non-magnetic member, and the portion of the nonmagnetic member in contact with the coil extending outward of the heating medium.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.